Cloning of chicken anaemia DNA

ABSTRACT

Double-stranded recombinant nucleic acids comprising a chicken anaemia virus (CAV)-specific nucleotide sequence, such as a CAV-specific nucleotide sequence consisting of the nucleotide sequence of FIG. 1 (SEQ ID NO: 1) and its complementary strand, are useful for preparation of diagnostics, vaccines and CAV-specific antibodies.

FIELD OF THE INVENTION

This invention is in the fields of genetic engineering (gene manipulation) by means of the recombinant DNA (and RNA) technology, diagnostics and immunization/vaccination. More in particular, the invention relates to the detection, cloning and sequence analysis of the Chicken Anaemia Virus (CAV) DNA genome and applications thereby made possible.

BACKGROUND OF THE INVENTION

The CAV virus that has not been classified so far causes infectious anaemia in chicken. The virus was first isolated in Japan in 1979 and was given its name because of the serious anaemia caused by it in young chicks (Yuasa et al, 1979). The other symptoms of CAV infection are the atrophy of the bone marrow and destruction of lymphocytes in the thymus. Lesions occur in the spleen and liver.

Day-old chicks are most susceptible. In these animals lethargy, anorexia and a passing aneamia are observed from 4 to 7 days after inoculation with CAV and about half of the animals die between 2 and 3 weeks after infection. With increasing age the natural resistance also increases. Upon infection at the age of seven days the chicks only develop a passing anaemia after infection, and upon infection of 14 days old animals no anaemia follows.

Protection against CAV infection and CAV disease symptoms is highly based on humoral immunological defence mechanisms. Vielitz (1989) developed a practical, rather effective method of prevention by means of a "controlled exposure" with CAV-infected liver suspensions in layers, the offspring thus acquiring maternal immunity. In Germany this method of immunization is used in practice, but it does not seem to be quite risk-free.

Animal experiments conducted in isolated poultry houses with the Centraal Diergeneeskundig Instituut (CDI) at Letystad have confirmed the protective value of maternal antibodies. Here the "controlled exposure" was carried out with CAV multiplied in tissue culture. The presence of maternal antibodies against CAV fully prevented the CAV replication upon infection of day-old chicks from thus vaccinated mother animals. The CAV symptoms did not occur either. This passive protection was also obtained in offspring of immunized layers and also after injection of specifically pathogen-free (SPF) chicks with yolk extracts of eggs of the same immunized layers. The passive protection with respect to CAV infection by means of administration of CAV antibodies lasted until the age of 4 weeks. Then the passive protection was found to be incomplete. These experiments showed that maternal antibodies produced by vaccination of mother animals will play an important preventive role in the practical situation.

It has also been demonstrated by way of experiment that in chicks that survive the CAV infection a passing depletion of a specific population of thymus lymphocytes occurs (Jeurissen et al, 1989). The thymus atrophy is the possible cause of the immunodepression causing CAV, resulting in that specific vaccinations are less effective, e.g. against Newcastle Disease. CAV has been isolated several times in flocks with increased losses owing to Marek's disease, Gumboro's disease (Infectious Bursal Disease Virus, IBDV; Yuasa et al, 1980) and in animals with Blue Wing Disease in association with reoviruses (Engstrom, 1988a, Engstrom et al, 1988b). With experimental double infections the enhancing properties of CAV with respect to other chicken viruses (e.g. Marek's Disease Virus, MDV, De Boer et al, 1989a) have been demonstrated. Recently a sharply increased inoculation reaction was observed in our own experiments after aerosol vaccination with Newcastle Disease vaccine and simultaneous CAV infection. CAV therefore leads to immunosuppressive and enhancing effects on other virus infections. These properties of CAV probably cause an increased incidence of virulent disease outbreaks in practice.

CAV seems to be spread all over the world. A considerable time after the CAV research had started in Japan the first CAV isolations were conducted in Europe, namely in Germany by Von Bulow (1983) and later by McNulty et al (1990) in the United Kingdom. In the Netherlands the first isolations of CAV from material from the USA, Israel and Tunesia were conducted by De Boer et al (1988). The available literature data indicate that the isolates belong to one serotype but several field isolates are to be tested for their mutual relationship and possible differences in pathogenicity (McNulty et al, 1990). The spread of CAV within a flock probably occurs by infection via feces and air. Vertical transmission of virus to the offspring, however, also plays an important role in CAV epidemiology. In various countries the presence of CAV was demonstrated seriologically.

Under tissue culture conditions CAV is hard to multiply. CAV hitherto causes only a cytopathologic effect (CPE) in MDV transformed lymphoblastoid cell lines from lymphomas of Marek's disease (MDCC-MSB1 cells) or Avian Leukaemia Virus (ALV) transformed lymphoblastoid cell lines from lymphold leukosis (1104-X5 cells; Yuasa, 1983).

A recent study by Todd et al (1990) describes virus particles (in purified CAV material) having a diameter of 23.5 nm which concentrate at a density of 1.33-1.34 g/ml in a CsCl gradient. The virus has one predominant polypeptide (Mr: 50,000) and a circular single-stranded DNA genome having a length of 2.3 kilobases. Two small viruses, the Porcine Circovirus and a virus associated with Psittacine Beak and Feather Disease, resemble CAV as regards the circular single-stranded DNA but have a smaller genome and a smaller virus particle diameter (Ritchie et al, 1989; Tischer etal, 1982). It was accepted for a long time that CAV belonged to the parvoviruses. Although most of the parvoviruses are single-stranded DNA viruses, they possess linear DNA, a larger genome and probably also another composition of viral polypeptides.

BRIEF DESCRIPTION OF THE INVENTION

It is generally accepted that cellular components involved in the replication and transcription of a virus are only functional if the DNA has a double-stranded form. A virus having a circular single-stranded DNA may occur in the cell in a phase in which it consists of double-stranded DNA. The present inventors have made use of this fact.

The present inventors have characterized the double-stranded CAV DNA having a length of 2.3 kilobase pairs in CAV-infected 1104-X5 and MDCC-MSB1 cells and cloned it in pIC-20H. The DNA was fully sequenced (see FIG. 1) (SEQ. ID. NO: 1). In a diagnostic test by means of labelled cloned CAV-DNA, CAV nucleic acids could be demonstrated in virus, liver and tissue culture preparations. Cloned CAV was bound to have all the biological and pathogenic properties of wild type CAV, both in tissue culture and in animal tests.

PCR and hybridization experiments showed that the cloned complete CAV genome is representative of CAV in the field. By means of Southern analyses with ³² P-labelled DNA probes it was demonstrated that all field isolates contained DNA molecules of 2.3 kb. Restriction enzyme analyses show that the cloned CAV DNA corresponds with the DNA of field isolates. In a dot blot assay it was demonstrated that with digoxigenin labelled cloned CAV DNA specifically hybridizes with DNA of the different field isolates. In PCR experiments using oligonucleotides the sequence of which was derived from the cloned CAV sequence (FIG. 4) (SEQ. ID. No: 2) CAV-DNA was specifically amplified or recognized.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides in a first aspect recombinant genetic information in the form of labelled or unlabelled DNA or RNA, comprising a Chicken Anaemia Virus (CAV) specific nucleotide sequence corresponding with or complementary to the nucleotide sequence of a CAV genome or part thereof.

A preferred embodiment of the present invention consists of such recombinant genetic information comprising a CAV-specific nucleotide sequence corresponding with or complementary to the nucleotide sequence shown in FIG. 1 (SEQ ID NO: 1), a nucleotide sequence homologous thereto to at least 60%, or part thereof.

This aspect of the invention consists of a nucleic acid selected from DNA and RNA, in any possible manifestation, i.e. both in the form of naked DNA or RNA and in the form of DNA or RNA packed in any way (i.e. in proteins or in virus particles) or connected with other matter (e.g., with a carrier or with a material functioning as a marker). The DNA may be both single-stranded and double-stranded DNA and may be both in linear and in circular form.

Characteristic of recombinant genetic information according to the invention is the presence therein of a CAV-specific nucleotide sequence. This CAV-specific sequence need not cover the entire genome of CAV and, from a practical point of view, only a specific part will be necessary and desirable for most of the applications.

A first preferred possibility is a CAV-specific nucleotide sequence corresponding with or complementary to a nucleotide sequence coding for a CAV protein and occurring in a CAV genome, or part thereof. Recombinant DNA comprising such a coding sequence may be used, e.g., for detecting CAV messenger RNA in a sample or may be used, e.g., within the scope of a process for producing CAV proteins or parts thereof. The words "part thereof" in principle comprise every part that can still be designated as CAV-specific. On a protein level this will be an epitope for most of the applications, i.e. an antigenic determinant recognizable by antibodies.

Another possibility is that the recombinant genetic information according to the invention comprises a CAV-specific nucleotide sequence corresponding with or complementary to a nucleotide sequence having a regulatory function, occurring in a CAV genome, or part thereof. One example is the use of CAV promoter/enhancer elements in combination with seauences coding for a protein other than CAV protein, e.g., to enable expression of such non-CAV proteins in poultry (such as chickens) and other animals in which the regulatory signals of CAV are effective.

Both in the above case and in general the recombinant genetic information according to the invention may also comprise a nucleotide sequence not derive d from a CAV genome. This "nucleotide sequence not derived from a CAV genome" may be formed by, e.g., a nucleotide sequence derived from a prokaryotic or eukaryotic expression vector. Thus, the invention comprises the possibility of an insertion of a CAV-specific sequence into a (vital or non-viral) vector suitable for expression in eukaryotic organisms or into a plasmid suitable for expression in bacteria. Furthermore, it is also possible that as "nucleotide sequence not derived from a CAV genome" recombinant genetic information according to the invention comprises a nucleotide sequence, not occurring in the CAV genome, having a regulatory function.

The "nucleotide sequence not derived from a CAV genome", however, may also consist of a nucleotide sequence coding for (part of) a protein other than a CAV protein, e.g., if CAV regulation signals are used to express such a non-CAV protein (or part thereof) in a host accessible to the CAV virus, or if the recombinant DNA is to be used to produce a hybrid or fusion protein in which a CAV protein functions as a carrier for an epitope of a non-CAV protein or, conversely, a non-CAV protein functions as a carrier for an epitope of a CAV protein.

If the recombinant genetic information according to the invention is to be used within the scope of processes for detecting complementary DNA or RNA in a sample, the presence of a label may be necessary. A label as used herein is a marker suitable for use with DNA or RNA which enables or facilitates detection of the labelled DNA or RNA. A person skilled in the art knows many types of markers suitable for this purpose, such as radioisotopes (e.g., hu 32P), enzyme molecules (e.g., peroxidases), haptens (e.g., biotin), fluorescent substances, dyes, pigments (e.g., inorganic phosphors), and particulate markers (e.g., gold or selenium particles).

In a second aspect the invention relates to the use of recombinant genetic information as defined above, in particular for diagnostic purposes, immunization or vaccination purposes, or for the production of CAV or non-CAV proteins.

More particularly, it concerns, e.g., a use of recombinant genetic information according to the invention as a CAV-specific probe or primer in a process for detecting CAV-DNA or --RNA, e.g. in a process of DNA/RNA slot blotting, Southern blotting, Northern blotting, in situ hybridization, DNA amplification by means of PCR, Sl mapping and primer extension, the invention also extending to a diagnostic kit for detecting CAV-DNA or --RNA in a process such as DNA/RNA slot blotting, Southern blotting, Northern blotting, in situ hybridization, DNA amplification by means of PCR, Sl mapping or primer extension, which diagnostic kit contains recombinant genetic information according to the invention as a CAV-specific probe or primer.

Further concerned is a use of recombinant genetic information according to the invention as a living virus vaccine to realize protection against CAV or another pathogen, the invention also extending to a vaccine preparation for immunizing against CAV or another pathogen, which preparation comprises recombinant genetic information according to the invention and optionally one or more carriers and adjuvants suitable for living virus vaccines.

Also concerned is a use of recombinant genetic information according to the invention as a cloning vector, i.e. a use of CAV-DNA as a kind of "eukaryotic plasmid" for avian systems in which gene fragments are incorporated into the complete or nearly complete CAV genome.

The use of recombinant genetic information according to the invention in a process for producing a CAV protein, part thereof or a protein other than a CAV protein, by in vitro or in vivo translation, is also comprised. The same applies to a prokaryotic or eukaryotic cell containing recombinant genetic information as defined above and, in particular, such a prokaryotic or eukaryotic cell capable of expression of at least one protein or protein part encoded by recombinant genetic information according to the invention. These different possibilities will be extensively explained lower down in this description.

A following aspect of the invention is concerned with CAV protein or part thereof obtained by in vitro translation of recombinant genetic information according to the invention, comprising a nucleotide sequence coding for the CAV protein or part thereof, as well as CAV protein or part thereof obtained by isolation from a prokaryotic or eukaryotic cell containing recombinant genetic information according to the invention comprising a nucleotide sequence coding for the CAV protein or part thereof and capable of expression thereof.

Also on the protein level the invention extends to the different applications, in particular the use of a CAV protein or protein part according to the invention for diagnostic purposes, immunization or vaccination purposes, or for the production of CAV-specific antibodies.

More in the concrete, the invention comprises the use of a CAV protein or protein part as defined above as a reagent binding CAV-specific antibodies in an immunoassay process for detecting CAV-specific antibodies, e.g., an immunoperoxidase staining, an ELISA or an immunofluorescence assay, and correspondingly a diagnostic kit for detecting CAV-specific antibodies in an immunoassay process such as an immunoperoxidase staining, an ELISA or an immunofluorescence assay, which diagnostic kit contains a CAV protein or protein part according to the invention as a reagent binding CAV-specific antibodies.

The invention also comprises :he use of a CAV protein or protein part as defined above as a subunit vaccine to realize protection against CAV, as well as a vaccine preparation against CAV, which preparation comprises a CAV protein or protein part according to the invention and optionally one or more carriers and adjuvants suitable for subunit vaccines.

The use of a CAV protein or protein part as defined above in a process for producing CAV-specific polyctonal or monoclonal antibodies also belongs to the possibilities falling within the scope of the invention. All these applications will be more extensively explained lower down in this description.

In a further aspect the invention also relates to CAV-specific antibodies produced by means of a CAV protein or protein part as defined above, as well as the different uses for such CAV-specific antibodies, e.g. for diagnostic purposes, immunization or vaccination purposes, or for preparative purposes.

More in the concrete, it concerns a use of CAV-specific antibodies according to the invention as a CAV protein binding reagent in an immunoassay process for detecting CAV protein, as well as a diagnostic kit for detecting CAV protein in an immunoassay process, which diagnostic kit contains CAV-specific antibodies according to the invention as CAV protein binding reagents.

A further example is a use of CAV-specific antibodies according to the invention for passive immunization against CAV infection, as well as an immunization preparation for passive immunization against CAV, which preparation comprises CAV-specific antibodies according to the invention and optionally one or more carriers and adjuvants suitable for passive immunization preparations. Specifically concerned is immunization of layers with recombinant products according to the invention.

As regards preparative applications, one example is the use of CAV-specific antibodies according to the invention in a process for isolating and/or purifying CAV protein. The most important uses will be explained more extensively in the following detailed description of the invention.

EXAMPLES

Analysis of low molecular DNA isolated from CAV-infected cells.

The CAV genome isolated from a purified virus preparation proved to be a circular single-stranded DNA molecule having a length of about 2300 bases (Todd et al, 1990). Our expectation was that in CAV-infected cells, in addition to circular single-stranded virus DNA, circular double-stranded CAV-DNA also occurs. Double-stranded DNA can be cut with restriction enzymes and therefore can be directly cloned, in contrast to single-stranded DNA. In view thereof, it was examined whether in the low molecular fraction of CAV-infected cells a DNA product occurs which was absent in uninfected cells.

Low molecular DNA was isolated from CAV-infected MDCC-MSB1 and 1104-X5 cells and from uninfected 1104-X5 cells. The DNA was fractionated on an agarose/ethidium bromide gel. A very weak DNA band having a (measured) length of about 3 kilobase pairs (kbp), was visible in the gel. This specific DNA product was absent in the DNA isolated from uninfected cells.

In the following experiment it was made more probable that the specific DNA was only present in CAV-infected cells. DNA isolated from infected cells was separated to length by means of an agarose gel. DNA having a length of 2.7-3.5 kbp was isolated. This DNA fraction will have to contain the specific virus DNA, in addition to other cellular DNA. The isolated DNA was radioactively labelled and hybridized with a Southern blot of low molecular DNA from CAV-infected cells and from uninfected cells. At the height of 3 kbp a DNA product hybridized in the blot of CAV-infected cells which was absent in the DNA blot of uninfected cells.

The length of 3 kbp was determined with DNA markers consisting of double-stranded linear DNA molecules. The behaviour of a circular double-stranded DNA molecule in an agarose gel is different from that of linear DNA fragments. The DNA of 3 kbp from CAV-infected cells could be a linear form of a DNA which, in reality, is 2.3 kbp in length.

If the circular double-stranded DNA is digested with a restriction enzyme cutting only once into the DNA molecule, a linear DNA molecule having a (measured) length of 2.3 kbp must be formed.

That this assumption is correct, was demonstrated by separatedly incubating low molecular DNA isolated from CAV-infected 1104-X5 cells with six different restriction enzymes (BamHI, EcoRI, HindIII, KpnI, PstI, and XbaI). A Southern blot of low molecular DNA isolated from CAV-infected 1104-X5 cells and cut with the above restriction enzymes was hybridized with the above radioactively labelled DNA probe. This showed that treatment with the restriction enzymes BamHI, EcoRI, Pstl, and XbaI resulted in a DNA molecule having a measured length of 2.3 kbp. DNA of uninfected cells incubated with BamHI did not contain this DNA product. The restriction enzyme HindIII cut twice into the DNA, while KpnI did not cut.

It can be concluded from the above experiments that in low molecular DNA of CAV-infected cells a 2.3 kbp circular DNA molecule occurs which is absent in uninfected cells and that this is the CAV genome in the form of a circular double-stranded DNA molecule. Cloning and subcloning of double-stranded CAV-DNA in a bacterial vector.

Low molecular DNA of CAV-infected 1104-X5 cells was separately incubated with BamHI, BcoRI, PstI, and XbaI. The DNA was seperated on a low melting point agarose gel. Of all four DNA preparations the 2.3 kbp DNA molecule was isolated. The cloning vector pIC-20H was separately digested with the same four restriction enzymes with which the low molecular DNA was cut. The linear vector was treated with "calf intestine alkaline phosphatase". Each 2.3 kbp DNA fragment was ligated at the corresponding restriction enzyme site of pIC-20H. The ligation products were transfected in the E. coli strain HB101. All 4 clonings gave plasmids containing inserted DNA having a length of about 2.3 kbp. A further restriction enzyme analysis showed that at least 7 plasmids contained the same DNA fragment. The place of integration of the vector, however, was different because of the use of different enzymes to cut open the circular molecule. By means of the restriction enzymes BamHI, EcoRI, PstI, and XbaI a restriction enzyme map was determined of all four CAV DNA-clones.

Four "different" CAV DNA plasmids were radioactively labelled and hybridized with Southern blots of BamHI-digested DNA isolated from CAV-infected and uninfected cells. All tested clones hybridized only with the 2.3 kbp DNA molecule present in DNA of CAV-infected cells.

Biological activity of two CAV DNA clones.

The two CAV clones pIC-20H/CAV-EcoRI and pIC-20H/CAV-PstI were digested with restriction enzymes so that the CAV DNA was entirely cut from the vector. The linear CAV DNA molecules were treated with T4-DNA ligase. The linear CAV DNAs were thus circuiarized. The "cloned" CAV DNA now had the double-stranded circular form also possessed by wild-type CAV DNA in infected cells. MDCC-MSB1 and 1104-X5 cells were transfected with the "cloned" circular CAV DNAs. For clone pIC-20H/CAV-EcoRI a very clear cytopathogenic effect (CPE) was found in both cell types. Clone pIC-20H/CAV-PstI caused a clear CPE in MDCC-MSB1 cells and a less clear CPE in 1104-X5 cells. However, the supernatants of pIC-20H/CAV-PstI transfected 1104-X5 cells caused a clear CPE in MDCC-MSB1 cells. Transfections with DNA isolated from CAV-infected cells also caused a clear CPE in MDCC-MSB1 cells, while in 1104-X5 cells a less clear CPE was to be seen. The CPE was not obtained after transfection of MDCC-MSB1 or 1104-X5 cells with pIC-20H vector DNA.

A Southern analysis showed that in cell lysates of MDCCMSB1 and 1104-X5 cells infected with virus (passage 6), obtained by cloned CAV DNA, CAV DNA was present.

A neutralization test with MDCC-MSB1 cells showed that the CPE caused by cloned DNA in the transfected cells was the result of a CAV infection. Neutralizing antibodies directed against CAV prevented the CPE of MDCC-MSB1 cells infected with CAV progeny of transfected cells.

Day-old chicks were injected intramuscularly with supernatant of transfected cells. In the chicken the supernatants caused the same clinical image as wild-type CAV: retarded growth appearing from differences in the total body weight, pale bone marrow and reduced hematocrit values (anaemia), thymus atrophy (depletion of a specific population of T cells) and mortality. Supernatants of cells transrecked with vector DNA caused no disease symptoms in the control chicks.

Sequence analysis of the double-stranded CAV DNA genome.

The entire double-stranded CAV DNA genome was completely sequenced by means of the Sanger method (Sanger et al, 1977) and the Maxam-Gilbert method. By means of the M13 sequencing and M13-reverse sequencing primers the DNA sequence of about 2100 bases was determined of the 4 pIC-20H/CAV (BamHI, EcoRI; PstI; XbaI) clones. Then the CAV genome was subcloned. Of the five different subclones of the CAV DNA genome the DNA sequence was determined by the Sanger method by means of the M13 primers and/or the Maxam-Gilbert method. Thus the DNA sequence of both strands of the CAV genome was determined.

The length of the CAV (double-stranded) DNA is 2319 bp. The first base of the EcoRI site of the circular CAV genome is numbered +1. The sequence of the DNA strand containing most of/the largest open reading frames is shown in FIG. 1 and is called (+) strand. The composition of the bases of this strand is: 25.5% adenine; 28.7% cytosine; 27.7% guanine; 18.1% thymine. Computer studies into possible homology of the CAV genome with already known virus sequences showed that the DNA was not described before and did not form part of an earlier described virus group. The inital hypothesis that CAV is a parvovirus is no longer sound as far as sequence and form of the CAV DNA genome (circular) are concerned.

By means of computer studies the organization of the CAV genome was characterized. The open reading frames, promoter/enhancer elements, polyadenylation signal and site, and "origin of replication" are predicted. FIG. 2 shows the predicted open reading frames, exceeding 300 bases, for both DNA strands of CAV. FIG. 2A shows the open reading frames beginning with the codon ATG. The ATG codon is the most frequently used initiation codon for proteins. It is remarkable that one of both DNA strands codes for 3 proteins having a length of 449 amino acids (51.6 kDa), 216 amino acids (24 kDa), and 121 amino acids (13.3 kDa). Todd et al (1990) showed a 50-kDa protein in purified CAV. If all the open reading frames are actually used, about 80% of the virus genome is translated into protein. Some regions even double. It is quite possible that the 3 open reading frames are translated from 1 RNA. The predicted start of the RNA molecule is at position 354 and the poly(A) addition at position 2317. The only poly(A) signal is at position 2287 of the plus strand.

It is unlikely that the open reading frames are used at the other DNA strand because this strand lacks some essential regulation sequences. FIGS. 2B and 2C show open reading frames using respectively CTG and GTG as a start codon. However, it is described for only a few proteins that these start codons are actually used (Hann at al, 1988).

Computer studies into similarities between the separate CAV proteins and already known proteins gave only limited homologies on sequences present in the available programmes. Accordingly, it is hard to predict what type of protein the CAV proteins resemble. A relatively high score was made by virai capsid, DNA-binding and blood coagulation proteins. The results are not given here.

The expression of proteins is regulated by promoter/enhancer elements (Jones, 1990). An eukaryotic promoter is mostly positioned right before the start of the transcript. The CAV sequence contains upstream of the cap site the general elements: TATA box, SP1 box, and CAAT box. The sequence and the position of these boxes excellently correspond with those described in most of the eukaryote promoters (Table 1). Around position 285 there may be binding sites for four different transcription factors: CREB, MLTF, GT, and PEA-I.

An eukaryote gene also contains enhancer elements determining the strength of the eukaryote promoter. Possible enhancer elements are the five direct repeats all having a length of 21 nucleotides and being located between positions 144 and 260. All repeats have 19 identical nucleotides. Only the last 2 nucleotides are different. Repeat 1 is identical with 2, and 3 is equal to 5. Repeats 1, 2, and 3 are located beside each other, like 4 and 5. Located between repeats 3 and 4 is a "break" of 12 nucleotides. A computer study shows that no (eukaryote) enhancer described contains all sequences found for the probable CAV enhancer elements. All direct repeats contain an ATF element which may be involved in the increase in the transcription of CAV RNAs. The direct repeats contain twice the sequence CATCC and twice the sequence CAGCC. The last sequence overlaps with the CAAT box. These four sequences only have 1 mismatch with the CACCC box described for β-globin (Table 1).

FIG. 3 shows that approximately between positions 55 and 135 (SEQ ID NO: 4-5) and between positions 2180 and 2270 (SEQ ID NO: 3) of the plus DNA strand very large hairpin structures are present in the (single-stranded) DNA form of CAV. Hairpin structures in the DNA may be involved in the replication of the CAV DNA. The hairpins between 2180 and 2270 may be present not only in CAV DNA but also in CAV RNA and are likely to play a role in the stability of the CAV RNA.

The different DNA forms of CAV in infected cells.

Four different CAV DNA molecules are visible in a Southern blot of a DNA preparation of CAV-infected cells. The DNA was hybridized with radioactively labelled DNA of clone pIC-20H/CAV-EcoRI. The CAV DNA molecules are, in view of their measured lengths and forms in a non-denaturing agarose gel and susceptibility to sl nuclease, respectively double-stranded open circles (3 kbp), supercoiled double-stranded DNA (2 kbp), circular single-stranded DNA (0.8 kbp) and single-stranded linear DNA (1.5 kbp). Sometimes the linear double-stranded DNA form of CAV is also visible (2.3 kbp). Todd et al (1990) have measured a length of 0.8 kbp for the circular single-stranded DNA from isolated CAV on the basis of the electrophoretic mobility in a non-denaturing agarose gel.

Detection of CAV DNA in virus preparation.

Total DNA was isolated from CAV and purified according to the method described by Von Bulow (1989). The DNA preparation was analyzed in a Southern assay with a labelled CAV DNA probe containing the entire cloned CAV sequence. DNA isolated from purified CAV contains a DNA molecule having a length of 0.8 kbp, measured in a non-denaturing agarose gel. In a Southern analysis of DNA isolated from purified CAV, with oligonucleotides derived from the cloned CAV DNA sequence as probes, it was demonstrated that the minus DNA strand is enclosed in the virus. From this it may be concluded that the single-stranded DNA of CAV in the capsid is the minus strand.

Southern analysis of DNA from CAV field isolates.

DNA preparations were prepared from CAV isolates obtained from chickens from flocks in which Marek's disease occurred to an increased extent. The DNA preparations from CAV isolates obtained in 12 companies in the Netherlands were collected a selectively from a collection of 60 samples. In only one company a higher mortality owing to Marek's disease was reported. Moreover, a CAV isolate originated from a guinea fowl. The CAV isolates examined by us were chiefly obtained after atrophy of the thymus was established upon examination by the Animal Health Services.

For the purpose of studying the degree of similarity between cloned CAV DNA (pIC-20H/CAV-EcoRI) and DNA of the different CAV field isolates MDCC-MSB1 cells were infected with the isolated CAV strains. A Southern analysis was conducted. All DNA preparations contained DNA molecules that specifically hybridized with ³² P-labelled cloned-CAV DNA. The DNA molecules of the different CAV field isolates have lengths corresponding with that of the cloned CAV and are double-stranded or single-stranded. Southern blot analyses directly conducted on tissue samples of the CAV-infected chickens from the field were found to contain DNA molecules that hybridized with labelled pIC-20H/CAV-EcoRI.

Restriction enzyme analysis of DNA from CAV field isolates.

The similarity of DNA from the different CAV field isolates with the cloned CAV genome was further examined by means of restriction enzyme analysis. The DNA preparations of the CAV isolates and of cloned CAV were separately cut with seven restriction enzymes. The enzymes BamHI, BglI, SstI, and XbaI proved to cut all DNAs identically. DNA of most of the field isolates contained two AccI sites and/or two HindIII sites, while DNA of only a few isolates contained the EcoRI site. FIG. 5 summarizes the restriction enzyme maps of the cloned CAV and the different field isolates. Per restriction enzyme site the number of field isolates containing the relevant site are bracketed.

Polymerase chain reaction (PCR) of DNA from CAV field isolates.

The oligonucleotides CAV-1 and CAV-2 (FIG. 4) (SEQ ID NO: 2) derived from the cloned CAV DNA sequence were synthesized. A PCR using these synthetic oligonucleotides was conducted to specifically detect DNA from CAV in the field. DNA isolated from MDCC-MSB1 cells infected with the different CAV isolates and DNA isolated from uninfected cells were amplified. After DNA amplification the DNA was electorphoretically separated to length on an agarose/ethidium bromide gel. An amplified 186 bp band (i.e. the value theoretically expected) was visible in all DNA samples of cells infected with the different CAV isolates. This specific band was not present after amplification of DNA isolated from uninfected cells. Amplified DNA bands of all field isolates show an identical rate of migration in the agarose gel. This result implies that no great deletions or insertions occur in this part of the genome of the different CAV field isolates. A Southern analysis with the ³² P-labelled oligonucleotide CAV-3 (FIG. 4) (SEQ ID NO: 2) showed that the 186 bp amplified DNA is CAV-specific and that no other DNA band hybridized with the CAV- 3 probe.

The susceptibility of detection of the CAV PCR was examined. DNA was isolated from CAV-infected cells, diluted stepwise, amplified and analyzed on an agarose/ethidium bromide gel. After amplification of samples containing an amount of DNA corresponding to the amount of DNA in about 100 CAV-infected cells, a CAV-specific DNA fragment of 186 bp was detected. However, if the amplified DNA was subjected to a Southern analysis with ³² P-labelled CAV-3 DNA, an amount of DNA corresponding to DNA from 1 cell was already found to result in a clearly visible CAV-specific DNA band. The CAV PCR is a very sensitive detection method which is specific for the hitherto examined CAV isolates.

Dot blot analysis of DNA from CAV field isolates with digoxigenin-labelled CAV DNA probes.

In addition to the PCR, an assay was developed for the detection of DNA from CAV field isolates. This test makes no use of radioactive probes. The CAV DNA insert of clone pIC-20H/CAV-EcoRI was labelled with 11-dUTP-digoxigenin. DNA preparations from MDCC-MSB1 cells, separately infected with the different CAV isolates, were blotted on a filter and analyzed for their ability to hybridize with the digoxigenin-labelled DNA probe. DNA preparations from MDCC-MSB1 cells infected with the different CAV isolates hybridized with the digoxigenin-labelled DNA probe, while DNA from uninfected cell cultures did not hybridize. This test using a nonradioactively labelled CAV DNA probe is therefore suitable for detection of DNA from CAV field isolates.

Applications

DNA.

CAV sequences of, e.g., the pIC-20H/CAV-EcoRI DNA plasmid or parts thereof can be used to demonstrate CAV DNA and/or RNA in preparations to be examined for research and diagnostics purposes. The DNA may be labelled radioactively or in another manner, e.g., with biotin/digoxigenin. By means of DNA/RNA slot blots, Southern/Northern analyses and in vitro hybridizations the presence of CAV nucleic acids can be established. Parts of the CAV sequences as used herein are also DNA oligomers.

Oligomers derived from the CAV sequences of clone pIC-20H/CAV-EcoRI can be used in a "Polymerase Chain Reaction" to trace very low concentrations of CAV DNA/RNA. The PCR is a very sensitive method frequently used for the detection of viruses.

Diagnostic kits based on the above applications are possible in practice.

For research purposes techniques like SI mapping and primer extension with the CAV DNA fragments are important. By these two methods, CAV RNA can be quantified and further characterized.

Oligomers in antisense configuration can be used to study gene functions. These may also serve as a model for studying novel methods of inhibiting virus replication.

CAV DNA may be used as a carrier in the transfection for small gene fragments, particularly if the pathogenic properties have been removed by deletion in the CAV genome.

CAV oligomers in antisense configuration may be expressed in virus vectors, which enables studying CAV replication or other gene functions in the living animal or in vitro.

RNA.

CAV DNA fragments cloned in Sp6/T7 vectors result in CAV RNA products. CAV RNAs obtained by in vitro transcription can be used for in vitro/in vivo synthesis of CAV proteins. Thus, RNA molecules, e.g. in a wheat germ extract, can be translated into proteins (in vitro translation). The CAV proteins obtained by in vitro translation may then be used, e.g., for tracing antibodies directed against CAV in sera of chickens (see below). CAV RNA molecules may also be forced into cells by micro-injection to be translated therein into proteins. Thus, the effects of CAV proteins can be studied on a cellular level. Protein/protein and/or protein/DNA interactions can also be analyzed.

CAV RNAs can also be used as probes for tracing CAV nucleic acids in preparations. The analyses can be conducted by means of slot blot, Southern, Northern and in situ hybridization analyses. These methods can be used to develop diagnostic tests for CAV.

Proteins.

All CAV proteins can be expressed in prokaryote or in eukaryote systems. This requires the CAV open reading frames found to be cloned in a suitable expression vector. For the bacterial system there is an expression vector based on the T7 promoter suitable for the expression of CAV open reading frames. The baculovirus system, yeast, and the CHO-dhfr system are possible eukaryote expression systems. Vital vectors, such as retrovirai vectors, are also eligible therefor.

The CAV proteins or epitopes located thereon can be used to trace antibodies directed against CAV. Thus, CAV-infected chickens can be traced. The CAV proteins or epitopes located thereon can be used in immunoassays, such as immunoperoxidase stainings, ELISAs and immunofluorescence assays.

The CAV proteins or epitopes located thereon can be used to provide humoral and/or cell-bound immunity against CAV. The CAV proteins obtained by expression in eukaryote and prokaryote vector/host systems can be used for use in subunit vaccines.

By means of the CAV proteins or epitopes located thereon CAV-specific antibodies can be obtained which enables CAV proteins to be traced in preparations of CAV-infected chickens (see below).

Antibodies.

In a number of infection tests in young chicks it could be confirmed that maternal antibodies-can provide effective passive protection against CAV infection. The maternal antibodies were transmitted to the young chicks via the natural route, as well as via injection of newly born .chicks with CAV antibody containing egg yolk extracts. Passive protection against a CAV infection was also provided by means of injection of egg yolk extracts of eggs from layers which had been infected with CAV just before the laying period.

Vaccination of layers with CAV proteins expressed in one of the above expression systems will result in the formation of maternal antibodies. Young chicks of these layers will be protected against CAV infection.

Diagnostic tests can be developed on the basis of antibodies against CAV. Both polyclonal and monoclonal antibodies may be used therefor. By means of CAV-specific antibodies, preparations can be examined for the presence of CAV proteins.

The above applications of CAV antibodies are possible for antibodies according to the invention, obtained by processes as described therein, in the same manner as for natural CAV antibodies.

Living virus vaccines.

Providing the immune system with viral proteins by means of a living virus vector is likely to result in a better immune response than a subunit vaccine. One or more CAV open reading frames (in whole or in part) could be cloned in living virus vectors. In poultry there can only be used living virus vectors that themselves show a good replication in the avian system. Eligible as vectors for application in the chicken are, e.g.: fowl pox virus, retrovital vectors, herpesvirus vectors (avian herpesvirus serotypes 1, 2, and 3) and infectious laryngotracheitis virus, and possibly also adenoviruses such as CELO. Immunization with the above living virus vectors protects against CAV and the carrier virus.

By means of applying one or more deletions in the CAV genome there may be developed vaccines that immunize against CAV infection in young chicks. When applying the deletions the pathogenic character of CAV infection must be eliminated but the replicarive and therefore immunizing properties must be retained.

The CAV genome can also itself be made suitable as a living virus vector for the expression of antigens of other viruses. This requires the CAV genome to be changed such that in addition to or instead of CAV proteins "foreign" virus proteins are expressed. CAV vectors therefore can be constructed such that protection occurs against "foreign" viruses alone or also against CAV, depending on the expression of the viral proteins by the recombinant vector in the vaccinated animal.

CAV vaccines produced as a subunit vaccine, a deletion vaccine or a gene fragment in another virus vector will chiefly be used for the vaccination of layers. However, vaccination of chicks at a younger age, e.g. in combination with a vaccination against Marek's disease, also remains a possible use of the invention.

Enhancer/promoter elements.

The CAV promoter and enhancer elements can be cloned in DNA vectors. Under the regulation of the CAV promoter/enhancer CAV proteins or "foreign" proteins can be expressed in both chicken cells and in other cell types.

It is conceivable that the CAV promoter is functional in (chicken) bone marrow cells. As a model system for gene therapy "foreign" proteins can be expressed in vitro in bone marrow cells by means of CAV promoter/enhancer elements, optionally in combination with retroviral vectors. The genetically modified bone marrow cells may then be transplanted into the bone marrow of, in the present case, the chicken. For very small gene fragments the CAV genome itself is also eligible for use as a vector.

The CAV enhancer/promoter elements could also be active in other organisms. If this should be the case, the elements can also be used in, e.g., the mouse system as a model for gene therapy.

Products of CAV itself under the regulation of our own CAV promoter or another promoter also provide possibilities for studying and developing techniques for gene therapy.

The possibility of using the entire or substantially the entire CAV genome as a cloning vector, i.e. as a kind of eukaryotic plasmid for avian systems, is a development that is to be considered real in view of the discovered structure of the CAV genome.

Depositing the CAV clone DIC-20H/CAV-EcoRI

A glycerol stock of HB101 cells transformed with the plasmid pIC-20H/CAV-EcoRI was deposited with the Centraalbureau voor Schimmelcultures at Baarn, The Netherlands, on Sep. 7, 1990, under number CBS 361.90.

DESCRIPTION OF THE FIGURES

FIG. 1 (SEQ ID NO: 1) shows the nucleotide sequence of the cloned CAV DNA. The total length is 2319 bases, the first G of the EcoRI site being taken as No. 1.

FIG. 2 shows the predicted open reading frames (ORFs) having a length of more than 300 bases for both DNA strands. The ORFs predicted for the 3 different start codons ATG, CTG and GTG are shown in the 3 SubFIGS. 2A, 2B and 2C, respectively.

FIG. 3A, 3B, and 3C (SEQ ID NO: 3-5) shows some predicted hairpin structures of the CAV genome consisting of single-stranded DNA.

FIG. 4 (SEQ ID NO: 2) shows the oligonucleotides used in the PCR. The DNA sequence and position of the oligonucleotides on the CAV genome are shown. The position of the nucleotides in the CAV genome corresponds with that shown in FIG. 1 (SEQ ID NO: 1).

FIG. 5 shows the restriction enzyme map of the cloned CAV DNA. Bracketed is the number of CAV isolates per total of 21 possessed by the relevant restriction enzyme site in the genome.

MATERIALS AND METHODS

Cell cultures and viruses.

The CAV isolates were cultured in transformed lymphoblastoid cell lines from rumours of chickens induced by the avian leukosis virus of subgroup A(1104-X-5) or by Marek's disease virus (MDCC-MSB1). The cell cultures were infected with about 0.1-1 TCID50 per cell. After two days the cells were harvested. The cells were infected with virus progeny of cloned CAV DNA, or field isolates. CAV-Cux-1, originally isolated in Germany from a flock of chickens suffering from Marek's disease (Von Bulow et al, 1983, 1985), was provided by Dr. M. S. McNulty, Veterinary Research Laboratories, Belfast, Northern Ireland. In January 1988, Dr. J. P. Rosenberger, University of Delaware, Newark, USA, sent us two blood samples for the determination of the virulence of the Marek's disease strain T-1704 and its derivative, MDV-Del-S which is the first passage in a chicken. We obtained the CAV-T-1704 and CAV-Del-S isolates from SPF-chickens infected with the MDV-strain T-1704 and its derivative MDV-Del-S. The Dutch CA isolates were aselectively selected from a series of sixty which were all cultured in MDCC-MSB1 cell cultures. The field material was supplied by J.C. van den Wijngaard, Gezondheidsdienst Brabant at Boxtel and J. Naber, Gezondheidsdienst voor Pluimvee at Doorn, mainly because atrophy of the thymus was established during autopsy. CAV isolates obtained from our own SPF flocks were added to the series.

Isolation of total DNA.

Virus and liver preparations were resuspended in 20 mM Tris HCl-pH 7.5, 2 mM EDTA, 0.2% SDS, 0.6 mg/ml Proteinase-K and incubated for 1 hour at 37° C. The preparations were extracted with phenol-chloroform-isoamyl alcohol (25:24:1), and the DNA was precipitated by means of ethanol. The DNA pellets were resuspended in 100 μl 10 mM Tris HCl-pH 7.5, 1 mM EDTA.

Extraction and analysis of low molecular DNA.

Low molecular DNA was isolated from CAV-infected 1104-X5 and MDCC-MSB1 cells and uninfected 1104-X5 cells according to the method described by Hirt (1967). The DNA was separated on agarose gels and, after staining with ethidium bromide, directly analyzed by means of UV light or blotted on a Biotrace filter according to the method described by Southern (1982).The blots were hybridized with random-primed ³² P-labelled DNA, isolated from low molecular DNA of CAV-infected 1104-X5 cells having a length of 2.7-3.5 kb.

Cloning of CAV DNA.

The entire CAV DNA genome was cloned in the bacterial vector pIC-20H. Partsof the CAV DNA genome were cloned in the vector pIC-19R. All plasmid DNA cloning steps were carried out in principle according to the methods described by Maniatis et al (1982).

Sequence analysis of CAV DNA.

CAV DNA plasmids were purified by means of a CsCl-gradient and Sephacryl-S500 (pharmacia) chromatography. Double-stranded DNA was sequenced by means of T₇ DNA polymerase (Pharmacia), or by means of Taq DNA polymerase (Promega). Both methods were conducted according to theinstructions given by Pharmacia or Promega. The oligonucleotides were kinated with T₄ nucleotide kinase of Pharmacia. "Strong stops" were sequenced according to the method described by Maxam and Gilbert (1977).

Circularization of the cloned CAV DNA genome.

10 μg plasmid DNA of clones containing the entire CAV DNA genome were digested with restriction enzyme so that the entire CAV DNA insert was separated from the vector DNA. T₄ -DNA ligase treatment of the 2.3 kilobase pairs of linear CAV DNA molecule resulted in a circular double-stranded CAV DNA. The ligation products were analyzed on a 0.8% agarose gel.

DEAE-dextran transfection.

For the transfection of ! 104-X5 and MDCC-MSB1 cells 2 μg religated CAV DNA were suspended twice in 25 μl Milli-Q water and mixed with 260 μl TBS buffer. 15 μl 10 mg/ml DEAE-dextran was added to the DNA mixture, and the mixture was incubated for 30 minutes at room temperature.

1104-X5 cells. A 50 mm tissue culture plate with 1-2×10⁶ 1104-X5cells/plate was washed twice with TBS buffer. The TBS buffer was completelyremoved from the cell monolayer, and 300 μl DEAE-dextran/DNA-dilution were added. The cells were incubated for 30 minutes at room temperature. The DEAE-dextran/DNA-mix was replaced by 2 ml 25% DMSO/TBS, and the cell monolayer was incubated for 2 minutes at room temperature. The cells were washed twice with TBS buffer, and then tissue culture medium (RPMI1640 or E-MEM) was added. The cells were incubated at 37° C.-5% CO₂.

MDCC-MSB1 cells. About 2×10⁶ MDCC-MSB1 cells were centrifuged at1500 rpm in a table centrifuge. The medium was replaced by 5 ml TBS buffer,and the cells were carefully resuspended. The washing step was repeated. All TBS buffer was removed, the cell pellet was carefully resuspended in 300 μl DEAE-dextran/DNA-mix and incubated at room temperature for 30 minutes. 0.5 ml 25% DMSO/TBS were added, and the suspension was incubated for 3 minutes at room temperature. 5 ml TBS were added, and the cells werecentrifuged at 1500 rpm in a table centrifuge. The supernatant was removed,and 50 ml tissue culture medium were added. The cells were resuspended and centrifuged off. The cells were received in 5 ml tissue culture medium andincubated at 37° C.-5% CO₂. By way of control, 2 μg pIc-20H plasmid were used for transfection.

In vitro neutralization test.

MDCC-MSB1 cells were infected with supernatant of MDCC-MSB1, and 1104-X5 cells were transfected with cloned "CAV DNA". About 2×10⁴ cellswere infected. The virus content of this inoculum was not exactly known. Inhalf of the infected cell cultures polyclonal serum having a neutralizing activity directed against CAV, diluted 1:100, was added to the medium. By way of control, a series of "wells" with CAV-infected MSB1 cells was takenalong, no antiserum directed against CAV being added to the medium.

CAV infection of day-old chicks.

Supernatants of CAV DNA and control DNA transfected MDCC-MSB1 and 1104-X5 cells were injected intramuscularly into day-old chicks. Six days after infection an autopsy was conducted at 5 chicks per group, after the hematocrit value and the total body weight had been determined first. For virus isolation and immunohistochemistry, heparin blood, thymus, and bone marrow were collected. The immunohistochemical research occurred by means of a peroxidase staining of thymus coupes with, inter alia, the CAV-specific monclonal CV1-85.1. Fourteen and twenty-eight days after infection an autopsy was conducted at 5 more chicks, and all the above determinations were carried out.

Polymerase chain reaction (PCR).

The oligonucleotides were synthesized by means of a Cyclone DNA synthesizer(Biosearch Inc. USA). The sequence was derived from the CAV DNA sequence shown in FIG. 1 (SEQ ID NO: 1). The PCR was isolated on DNA from CAV-infected and uninfected MDCC-MSB1 cells. The final concentration of the reagents were: 50 mM KCl, 10 mM Tris-HCl (pH 8.3), 3 mM MgCl₂, 0.01% calf serum albumin, 200 μM of each dNTP, 1 μM of each oligonucleotide and 2 units of Tag-DNA polymerase (CETUS, USA) in total 100 μl. The DNA samples were cyclically incubated 30 times at 93° C for 1 minute, at 55° C for 1 minute, and at 72°C for 3 minutes in a Perkin Elmer/Cetus thermal cycler. One tenth of the amplified DNA was directly analyzed on a 2% agarose/ethidium bromide gel, or by Southern blot analysis. The DNA probe used was the oligonucleotide that was terminally labelled with ³² P according to Maniatis et al (1982).

Dot blot analysis

The CAV DNA insert of pIC-20H/CAV-EcoRI was isolated and labelled with digoxigenin-11-dUTP (Boehringer, Mannhelm, Germany) according to the protocol of the supplier. Biotrace-RP filters were saturated with 1.5M NaCl and 0.15M Na citrate. The DNA samples were resuspended in 10 mM Tris HCl (pH 7.5) and 1 mM EDTA, boiled for 3 minutes, cooled on ice and placedon the filter. The filter was dried at room temperature and incubated for 30 minutes at 65° C. The filters were hybridized with digoxigenin-labelled DNA. The DNA labelled with digoxigenin was made visible by means of an immunological staining according to the protocol ofthe supplier.

                                      TABLE 1                                      __________________________________________________________________________     Known transcription factor binding sequence elements in the                    enhancer/promoter region of CAV.                                                         Consensus           CAV                 Position in                  Element   sequence            sequence            CAV sequence                 __________________________________________________________________________     TATA-#    GTATA/.sub.T A.sup.A /T                                                                            GTATATAT            321-330+                       SP1     GGGCGG              GGGCGG              305-310+                       CREB    TGACGTCA            TGACGT              290-297                        PEA-I.sup.(Py)                                                                         GGAAGTGACTAAC (SEQ ID NO: 6)                                                                       GAAAGTGACTTTC (SEQ ID NO:                                                                          286-298                        GT.sup.(SV40)                                                                          G.sup.G /.sub.C TGTGGAA.sup.A /GT (SEQ ID NO:                                                      CGTTGCGAAAGT (SEQ ID NO: 9)                                                                        279-290                        MLTF    GGCCACGTGACC (SEQ ID NO: 10)                                                                       TGCCACTGTCGA (SEQ ID NO: 11)                                                                       274-285                        CCAAT-TF                                                                               AGCCAAT             AGCCAAT             260-266+                     CACCC-#                                                                          CACCC   CAGCC               259-263                                            ATF     ACGTCA              ACGTCA              253-258+                     10.                                                                            CACCC-#                                                                          CACCC   CAGCC               236-240                                            ATF     ACGTCA              ACGTCA              232-237+                       SP1.sup.(weak)              GAGGCG              209-214                        ATF     ACGTCA              ACGTCA              199-204+                     CACCC-#                                                                          CACCC   CATCC               182-186                                            ATF     ACGTCA              ACGTCA              178-183+                     CACCC-#                                                                          CACCC   CATCC               161-165                                            ATF     ACGTCA              ACGTCA              157-162+                     __________________________________________________________________________     - CAP site is probably at about 350                                             + perfect homology between CAV and consensus sequence                            consensus sequence found in several viruses                                  # DNA sequence of an element                                             

References

1. De Boer, G. F., Pol, J. M. A., and Jeurissen, S. H. M. (1988). Marek's disease vaccination strategies using vaccines made from three avian herpesvirus serotypes. Proceedings First International Poultry and PoultryDiseases Symposium, Manisa, Turkey, pp. 38-48 (in Turkish, English abstract).

2. De Boer, G. F., Jeurissen, S. H. M., Van Roozelaar, D. J., Vos, G. J., and Koch, G. (1989a). Enhancing effects of chicken anaemia agent (CAA) on Marek's disease pathogenesis. Proceedings of the Thirty-eighth Western Poultry Disease Conference, Tempe, Arizona, U.S.A, p. 28.

3. De Boer, G. F., Pol, J. M. A., and Jeurissen, S. H. M. (1989b). Enhancing effects of chicken anaemia agent (CAA) on Marek's disease pathogenesis. Abstractbook IXth Intern. Congress of the WVPA, Brighton, U.K., p. 74.

4. Engstrom, B. E. (1988). Blue wing disease of chickens: Isolation of avian reovirus and chicken anaemia agent. Arian Pathology 17, 23-32.

5. Engstrom, B. E., Fossum, O., and Luthman, M. (1988). Blue wing disease of chickens: Experimental infection with a Swedish isolate of Chicken Anaemia Agent and an avian Reovirus. Avian Pathology 17, 33-50.

6. Hann, S. R., King, M. W., Bentley, D. L., Anderson, C. W., and Eisenman,R. N. (1988). A non-AUG translational initiation in c-myc exon 1 generates an N-terminally distinct protein whose synthesis is disrupted in Burkitt'slymphomas. Cell 52, 185-195.

7. Hirt, B. (1967). Selective extraction of polyoma DNA from injected mousecell cultures. Journal of Molecular Biology 26, 365-369.

8. Jeurissen, S. H. M., Pol, J. M. A., De Boer, G. F. (1989). Transient depletion of cortical thymocytes induced by chicken anaemia agent. Thymus 14, 115-123.

9. Jones, N. (1990). Structure and function of transcription factors. Seminars in Cancer Biology 1, 5-19.

10. Maniatis, T., Fritsch, E. F., and Sambrook, J. (1982). Molecular Cloning: A Laboratory Manual. New York: Cold Spring Harbor Laboratory.

11. Maxam, A. M., and Gilbert, W. (1977). A new method for sequencing DNA. Proceedings National Academic Sciences U.S.A. 74, 560-564.

12. McNulty, M. S., Connor, T. J., McNeilly, F., McLoughlin, M. F., and Kirkpatrick, K. S. (1990). Preliminary characterisation of isolates of chicken anaemia agent from the United Kingdom. Avian Pathology, In Press.

13. Ritchie, B. W., Niagro, F. D., Lukert, Steffens, W. L., and Latimer, S.(1989). Characterization of a new virus from cockatoos with psittacine beakand feather disease. Virology 171, 83-88.

14. Sanger, F., Nicklen, S , and Coulsen A R (1977) DNA sequencing with chain-terminating inhibitors. Proceedings National Academic Sciences U.S.A. 74, 5463-5467.

15. Southern, E. M. (1975). Detection of specific sequences among DNA fragments separated by gel electrophoresis. Journal of Molecular Biology 98, 503-517.

16. Tischer, I., Gelderblom, H., Vetterman, W., and Koch, M. A. (1982). A very small porcine virus with circular, single-stranded DNA. Nature 295, 64-66.

17. Todd, D., Creelan, J. L., Mackie, D. P., Rixon, F., and McNulty, M. S. (1990). Purification and biochemical characterisation of chicken anaemia agent. Journal of General Virology 71, 819-823.

18. Vielitz, E. (1989). Protect your chicks from infectious anemia. PoultryScience 68, 34-35.

19. Von Bulow, V., Fuchs, B., Vielitz, B., and Landgraf, H. (1983). Frusterblichkeitssyndrom bei Kuken nach Doppelinfection mir dem Virus des Marekschen Krankheit (MDV) und eine Anamie-Erreger (CAA). Zentralbiatt furVeterinarmedizin B. 30, 742-750.

20. Von Bulow, V., Fuchs, B., and Bertram, M. (1985). Untersuchungen uber den Erreger der Infectiosen Anamie-Erreger bei Huhnerkucken (CAA) in vitro: Vermehrung, Titration, Serum-neutralisationstest und indirekter Immunofluoreszenstest. Zentralblatt fur Veterinarmedizin B. 32, 679-693.

21. Yuasa, N., Taniguchi, T., and Yoshida, I. (1979). Isolation and some properties of an agent inducing anaemia in chicks. Avian Diseases 23, 366-385.

22. Yuasa, N., Taniguchi, T., and Yoshida, I. (1980). Effect of infectious bursal disease virus infection on incidence of anaemia by chicken anaemia agent. Avian Diseases 24, 202-209.

23. Yuasa, N. (1983). Propagation and infectivity titration of the Gifu-1 strain of chicken anaemia agent in a ceil line (MDCC-MSB1) derived from Marek's disease lymphoma. National Institute of Animal Health Quarterly 23, 13-20.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 11                                                  (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 2319 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: circular                                                         (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        GAATTCCGAGTGGTTACTATTCCATCACCATTCTAGCCTGTACACAGAAAGTCAAGATGG60                 ACGAATCGCTCGACTTCGCTCGCGATTCGTCGAAGGCGGGGGGCCGGAGGCCCCCCGGTG120                GCCCCCCTCCAACGAGTGGAGCACGTACAGGGGGGTACGT CATCCGTACAGGGGGGTACG180               TCATCCGTACAGGGGGGTACGTCACAAAGAGGCGTTCCCGTACAGGGGGGTACGTCACGC240                GTACAGGGGGGTACGTCACAGCCAATCAAAAGCTGCCACGTTGCGAAAGTGACGTTTCGA300                AAATGGGCGGCGCAA GCCTCTCTATATATTGAGCGCACATACCGGTCGGCAGTAGGTATA360               CGCAAGGCGGTCCGGGTGGATGCACGGGAACGGCGGACAACCGGCCGCTGGGGGCAGTGA420                ATCGGCGCTTAGCCGAGAGGGGCAACCTGGGCCCAGCGGAGCCGCGCAGGGGCAAGTAA T480               TTCAAATGAACGCTCTCCAAGAAGATACTCCACCCGGACCATCAACGGTGTTCAGGCCAC540                CAACAAGTTCACGGCCGTTGGAAACCCCTCACTGCAGAGAGATCCGGATTGGTATCGCTG600                GAATTACAATCACTCTATCGCTGTGTGGCTGCG CGAATGCTCGCGCTCCCACGCTAAGAT660               CTGCAACTGCGGACAATTCAGAAAGCACTGGTTTCAAGAATGTGCCGGACTTGAGGACCG720                ATCAACCCAAGCCTCCCTCGAAGAAGCGATCCTGCGACCCCTCCGAGTACAGGGTAAGCG780                AGCTAAAA GAAAGCTTGATTACCACTACTCCCAGCCGACCCCGAACCGCAAAAAGGCGTA840               TAAGACTGTAAGATGGCAAGACGAGCTCGCAGACCGAGAGGCCGATTTTACTCCTTCAGA900                AGAGGACGGTGGCACCACCTCAAGCGACTTCGACGAAGATATAAATTTCGA CATCGGAGG960               AGACAGCGGTATCGTAGACGAGCTTTTAGGAAGGCCTTTCACAACCCCCGCCCCGGTACG1020               TATAGTGTGAGGCTGCCGAACCCCCAATCTACTATGACTATCCGCTTCCAAGGGGTCATC1080               TTTCTCACGGAAGGACTCATTCTGCC TAAAAACAGCACAGCGGGGGGCTATGCAGACCAC1140              ATGTACGGGGCGAGAGTCGCCAAGATCTCTGTGAACCTGAAAGAGTTCCTGCTAGCCTCA1200               ATGAACCTGACATACGTGAGCAAAATCGGAGGCCCCATCGCCGGTGAGTTGATTGCGGAC1260                GGGTCTAAATCACAAGCCGCGGACAATTGGCCTAATTGCTGGCTGCCGCTAGATAATAAC1320              GTGCCCTCCGCTACACCATCGGCATGGTGGAGATGGGCCTTAATGATGATGCAGCCCACG1380               GACTCTTGCCGGTTCTTTAATCACCCAAAGCAGATGACCCTGCA AGACATGGGTCGCATG1440              TTTGGGGGCTGGCACCTGTTCCGACACATTGAAACCCGCTTTCAGCTCCTTGCCACTAAG1500               AATGAGGGATCCTTCAGCCCCGTGGCGAGTCTTCTCTCCCAGGGAGAGTACCTCACGCGT1560               CGGGACGATGTTAAGTACA GCAGCGATCACCAGAACCGGTGGCAAAAAGGCGGACAACCG1620              ATGACGGGGGGCATTGCTTATGCGACCGGGAAAATGAGACCCGACGAGCAACAGTACCCT1680               GCTATGCCCCCAGACCCCCCGATCATCACCGCTACTACAGCGCAAGGCACGCAAGTCCGC 1740              TGCATGAATAGCACGCAAGCTTGGTGGTCATGGGACACATATATGAGCTTTGCAACACTC1800               ACAGCACTCGGTGCACAATGGTCTTTTCCTCCAGGGCAACGTTCAGTTTCTAGACGGTCC1860               TTCAACCACCACAAGGCGAGAGGAGCCGGGGACCCCA AGGGCCAGAGATGGCACACGCTG1920              GTGCCGCTCGGCACGGAGACCATCACCGACAGCTACATGTCAGCACCCGCATCAGAGCTG1980               GACACTAATTTCTTTACGCTTTACGTAGCGCAAGGCACAAATAAGTCGCAACAGTACAAG2040               TTCGGCACAGC TACATACGCGCTAAAGGAGCCGGTAATGAAGAGCGATGCATGGGCAGTG2100              GTACGCGTCCAGTCGGTCTGGCAGCTGGGTAACAGGCAGAGGCCATACCCATGGGACGTC2160               AACTGGGCGAACAGCACCATGTACTGGGGGACGCAGCCCTGAAAAGGGGGGGGGG CTAAA2220              GCCCCCCCCCCTTAAACCCCCCCCTGGGGGGGATTCCCCCCCAGACCCCCCCTTTATATA2280               GCACTCAATAAACGCAGAAAATAGATTTATCGCACTATC2319                                    (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                   (A) LENGTH: 210 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        ACCGGTCGGCAGTAGGTATACGCAAGGCGGTCCGGGTGGATGCACGGGAACGGCGGACAA60                 CCGGCCGCTGGGGGCAGTGA ATCGGCGCTTAGCCGAGAGGGGCAACCTGGGCCCAGCGGA120               GCCGCGCAGGGGCAAGTAATTTCAAATGAACGCTCTCCAAGAAGATACTCCACCCGGACC180                ATCAACGGTGTTCAGGCCACCAACAAGTTC 210                                             (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 82 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        GTACTGGGGGACGCAGCCTGAAAAGGGGGGGGGGTAAACCCCCCCCCCTTAAA CCCCCCC60                CTGGGGGGGATTCCCCCCAGAC82                                                       (2) INFORMATION FOR SEQ ID NO:4:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 36 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                            (ii) MOLECULE TYPE: DNA (genomic)                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                        TGGACGAATCGCTCGACTTCGCTCGCGATTCGTCGA36                                         (2) INFORMATION FOR SEQ ID NO:5:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 46 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D ) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                        TCGAAGGCGGGGGGCCGGAGGCCCCCCGGTGGCCCCCCTCCAACGA46                               (2) INFORMATION FOR SEQ ID NO:6:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 13 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                        (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                        GGAAGTGACTAAC13                                                                (2) INFORMATION FOR SEQ ID NO:7:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 13 base pairs                                                      (B) TYPE: nucleic acid                                                          (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                        GAAAGTGACTTTC13                                                                (2) INFORMATION FOR SEQ ID NO:8:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 12 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                        GSTGTGGAAWGT12                                                                 (2) INFORMATION FOR SEQ ID NO:9:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 12 base pairs                                                       (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                        CGTTGCGAAAGT12                                                                 (2) INFORMATION FOR SEQ ID NO:10:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 12 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                                       GGCCACGTGACC12                                                                 (2) INFORMATION FOR SEQ ID NO:11:                                              (i) SEQUENCE CHARACTERISTICS:                                                   (A) LENGTH: 12 base pairs                                                     (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                                       TGCCACTGTCGA12                                                             

We claim:
 1. An isolated and purified double-stranded recombinant nucleic acid comprising a chicken anemia virus (CAV)-specific nucleotide sequence consisting of the nucleotide sequence shown in FIG. 1 (SEQ ID NO: 1) and its complemetary strand.
 2. An isolated and purified recombinant nucleic acid according to claim 1, wherein said CAV-specific nucleotide sequence codes for a CAV protein.
 3. An isolated and purified recombinant nucleic acid according to claim 1, wherein said CAV-specific nucleotide sequence has a regulatory function.
 4. An isolated and purified recombinant nucleic acid according to claim 1, further comprising a nucleotide sequence not derived from a CAV genome.
 5. An isolated and purified recombinant nucleic acid according to claim 1, wherein said nucleic acid is labeled with one or more markers suitable for labeling DNA.
 6. An isolated and purified recombinant nucleic acid according to claim 5, wherein said markers are selected from the group consisting of radioisotopes, enzymes, haptens, fluorescent substances, dyes, and pigments.
 7. An isolated prokaryotic or eukaryotic cell containing an isolated and a recombinant nucleic acid according to claim
 1. 8. A prokaryotic or eukaryotic cell according to claim 7 which expresses at least one protein or protein part encoded by said recombinant nucleic acid.
 9. A method for expressing protein in avian systems comprising transfecting an avian system with a vector comprising an isolated and purified recombinant nucleic acid in accordance with claim
 1. 